When, e.g., according to the process of parent application Ser. No. 606,785(a) SO.sub.2 -containing waste gas is scrubbed with circulating calcium-ion-containing clear wash liquid having a maximum initial pH of 12 and a minimum subsequent pH in the weakly-acid zone, (b) acid or salt ingredient means are then added to the gas-contacted wash liquid to increase the degree of calcium-ion dissociation therein, (c) calcium bisulfite in the ingredient-means-containing wash liquid is subsequently oxidized to calcium sulfate at a pH of at least 4.5, (d) one or more calcium-ion-providing compounds, e.g. calcium carbonate, calcium oxide and calcium hydroxide, are thereafter added to the wash water and (e) the resulting calcium-ion-containing wash liquid is clarified before scrubbing SO.sub.2 -containing gas therewith, comparatively large quantities of wash liquid and correspondingly large equipment are required to effect any particular degree of SO.sub.2 removal from the SO.sub.2 -contaminated gas being scrubbed.
The so-called "water factor" is the volume of water (in liters) which is required in the scrubber (washer) to remove 90% (by volume) of the SO.sub.2 from 1 Nm.sup.3 (one cubic meter at atmospheric pressure and 20 degrees centigrade) of gas, e.g. waste gas or flue gas, containing 2.5 grams (g) per Nm.sup.3 of SO.sub.2. Even when the means to increase the calcium-ion-dissociation includes both strong inorganic, e.g. hydrochloric, acid or salt thereof and monobasic organic acid, such as formic acid, or a salt thereof, the water factor is 2.8 liters per Nm.sup.3 of moist flue gas. Ever-increasing requirements for purifying gases, particularly for removing harmful or noxious substances from flue gas, render extraordinarily significant any material reduction in the water factor and concomitant reduction in operation and equipment costs.